1. Field of the Invention
The present invention relates to telephone signaling, and more particularly, to a device for translating channel-associated signaling, of the type wherein the signaling bits for each data channel are associated with each other and with their corresponding data, into SONET-formatted signaling.
2. Description of the Prior Art
The American National Standard Institute, Inc. (ANSI) T1.105-1988 describes the Synchronous Optical Network (SONET) protocol for telecommunications equipment. This standard is incorporated herein by reference. The SONET protocol is particularly adapted for optical transmission, and various transmission levels have been standardized at specified line rates in M bit/s. The first level, Optical Carrier Level 1, or OC-1, transmits data at the rate of 51.84 M bits/s. This carrier level has a corresponding electrical level called Synchronous Transport Signal Level 1, or STS-1.
In order to access this high-frequency carrier level, access products are required so that lower bandwidth carriers can be introduced into or extracted from the STS-1 transmission level. These access products provide a SONET network with nodes where components of an STS-1 signal can be added to or dropped out of the main signal. The components that are extracted must be reorganized to produce a signaling format compatible with currently-used telephone standards. In like manner, components that are added must have their signaling reorganized for insertion into the SONET format. A typical sub-component of an STS-1 signal would be a DS1 signal having a bit rate of 1.544 M bits/s. Twenty-eight DS1 signals can be supported by an STS-1 carrier. Within the DS1 signal format, an additional 24 DS0 64 K bits/s signals can be supported.
The SONET transmission is serial, comprising a total of 810 bytes. The frame structure for an STS-1 is shown in FIG. 1. The frame comprises 90 columns.times.9 rows of bytes, with 8 bits per byte. The sequence of transmission of the bytes is row by row, from left to right. The frame is divided into three parts: the section and line overhead, which are contained in the first three columns; and the payload, which is found in the 87 remaining columns, which, in connection with the nine rows, forms a Synchronous Payload Envelope, SPE, which includes 783 bytes. Nine of the SPE bytes are allocated to path overhead. The SPE can begin anywhere within the 87.times.9 byte envelope. Typically, the SPE begins in one SONET frame and ends in another. A payload pointer provided in overhead bytes H1 and H2 points to the byte where the SPE begins, shown as P=0 in FIG. 1. The information within the SPE is transported in Sub-STS-1 payloads called Virtual Tributaries, or VTs. There are several levels of VTs; however, it is only necessary to deal with a VT 1.5 for purposes of describing this invention. When the STS-1 payload supports 28 DS1 services, one VT at the 1.5 level is provided for each DS1 service.
FIG. 2 illustrates the payload mapping of SONET bytes into a DS1.
An SPE consists of 783 bytes belonging to 28 tributaries, wherein each tributary can carry a DS1 payload, as illustrated in FIG. 2. A DS1 payload has 27 bytes, 24 of which carry DS0 channels. The first byte carries a VT pointer, or address; a second byte is unused; and the third byte carries signaling data for the DS1 payload. Every channel has four signaling bits, namely, A, B, C and D, as is well known in the telephony art. Thus, for a DS1 payload of 24 channels, a total of 96 signaling bits are required. Since only four bits of signaling are carried in each SONET signaling byte and there is only one signaling byte per tributary or DS1, a total of 24 SONET frames would be required in order to transmit the 96 required signaling bits.
FIG. 3 illustrates the transmission order of the payload contained within the SPEs of 24 SONET frames. For the sake of clarity, a number of bytes of the SPE are now shown in FIG. 3. These bytes include: the first two rows of each SPE, which would contain bytes 1 and 2 of each of the 28 tributaries, as shown in FIG. 2, the nine path overhead bytes, and additional `fixed stuff` bytes. In addition, the SPE has been shown as being located entirely within one SONET frame. This facilitates the illustration in FIG. 3 of the signaling bits provided in each SPE byte. Thus, the first row of each frame shown in FIG. 3 is the signaling row and contains byte number 3 for each of the tributaries. The transmission order proceeds from left to right in each descending row of a frame. Thus, bytes containing four signaling bits for tributaries 0-27 are transmitted in sequence, after which the data for channels 0 for each tributary is transmitted, followed by the data for the other channels through to the transmission of the data for channels 23.
Due to the presence of nine overhead bytes (not shown), bytes 1 and 2 of each tributary, and additional unused `fixed stuff` bytes in the SPE, the signaling bytes start with SPE byte 60 and continue through byte 87. The content of each SONET signaling byte is as follows:
______________________________________ (MSB) (LSB) ______________________________________ Bit No. 7 6 5 4 3 2 1 0 Byte Sync R R Sl S2 S3 S4 F R Bit Sync 1 0 R R R R F R ______________________________________ R bits are not used
In the above, S1, S2, S3 and S4 are the signaling bits corresponding to the sets of four bits shown in the signaling bytes in FIG. 3. Thus, the signaling bits transmitted in the SONET signaling rows of sequential frames are transmitted in the order of all A bits, all B bits, all C bits and all D bits, which bits are not associated with their corresponding channel data and the A, B, C and D bits from a channel are not associated with each other.
Commonly used telephony signaling systems include systems wherein the signaling bits are transmitted inband within the channel data, and other systems transmit the signaling bits in separate signaling channels. However, in most cases the signaling bits for a channel are associated with each other or are transmitted in association with the channel data. Thus, a system was required which could extract the signaling information from lower-level telephone transmission lines and reassemble the signaling bits into a format which would facilitate insertion of the bits into the SONET format shown in FIG. 3. A total of 2,688 signaling bits must be transmitted on the SONET STS-1 carrier, and these bits must be accumulated from the 24 channels of each of the 28 DS1 carriers supported by the STS-1 transmission level, with each channel having A, B, C and D signaling bits. There are no known solutions to the problem of translating signaling data from the standard signaling formats into the SONET format.
In the system for which this invention was designed, a 16-bit internal byte was used for each channel, with each bit being provided on a separate line of a parallel bus. Thus, for each clock pulse a complete byte of channel information was obtained. The internal signaling format for a 16-bit byte is compared to a SONET data byte as follows: ##STR1## It is to be noted that the internal byte includes a full eight bits of data, which bits are directly transferable to a SONET data byte. The signaling bits A, B, C and D are located out of the data band and therefore do not deteriorate the transmitted data.
The STS-1 SONET format handles 672 data channels, each with ABCD signaling, for a total of 2,688 signaling bits. These bits are available in every internal 125 u sec frame time and are provided in the four outband bit positions. These signaling bits, provided in the same byte as the internal data, must be extracted from the internal byte and rearranged for insertion into the SONET format.
The access products used to extract and add sub-components to the STS-1 transmission level include both an add-drop multiplexer and a terminal multiplexer. The terminal multiplexer receives and extracts all data from the STS-1 and inserts new data in a return path. The add-drop multiplexer, however, poses special problems, since it facilitates the extraction and/or addition of any number of channels carried on the STS-1 line. Thus, some channels pass directly through an add-drop multiplexer, while some channels are extracted and other channels are added. All received channel signaling is converted to the internal format. Thus, the through channels could be treated similarly to the add channels by reconstructing the SONET signaling format from the outband signals provided in the internal format. However, this would cause unnecessary delay for through channels, as the signaling is processed by the access product. Preferably this delay should be avoided by passing each tributary signaling byte through the multiplexer and only overwriting the signaling bits that are being added.